Of the three that you mentioned, I like the Chrosziel MagNum for a lot of different reasons. I own two FIZ systems, and I like the MagNum more than my Preston because of both ergonomics and features.
the hand unit of really is ergonomic, lightweight, and durable and can be adjusted for left and right handed pullers. there are two hand units, both the same style, one is more advanced and offers adjustable tension on the wheel along with a variety of other features. there are a handful of accessories available like a lemo connector light and belt clip which I have found to be helpful as I have had several different FIZ units dropped by ACs slinging the hand unit on to a c-stand knob, but the belt clip makes things convenient. there are a host of user buttons that are assignable to things like light on/off, zoom positions, focus ranges, etc.
can work on GHz or MHz and even displays the ongoing activity on each of the channels so that you can choose your channel accordingly.
expandable to 3 motors (maybe 4 for 3D?) and can even run a serial connection to cabrio lenses that will run the servos which means that motors are not required on the camera build (this is useful for zoom and iris but focus is limited to the speed of the servo which is a little slow). has a cold shoe mount that allows a variety of mounting positions and even mounts in the T-bracket for GPI Pro rigs natively. The kit also comes with combined lightweight-studio mount that attaches to your 15mm and/or 19mm rods and lives on the camera like an FF4. The MN is thinner than a Preston 3 MDR, lighter weight, and easier to mount.
reverse polarity protection, electronic and mechanical fuse
you can use pretty much any motor on this system. i have one of the chrosziel cdm motors which is perfect for gimbal work and great on lenses like the cabrio 19-90, andgeniuex zooms, and any lens that isnt too tight. i have 3 heden motors that work seamlessly with the receiver. i have attached my DM1 motors without a problem. the MagNum is sold with or without Heden and Betz motors.
cinetape readout is large and easy to integrate.
i havent had any negative experiences with the system. it is a little bit techy and I have been confused about motor setup in the handset, but Timm at Chrosziel quickly explained it out to me.
portability in a small case is very easy with this system. the hand unit is just one piece rather than a hand unit, plus transmitter, plus zoom handle, no cables connecting parts. the components are lightweight, robust and i think that this fits well with large and small hands. if you really like to get into the system, there are a lot of custom options for pulling, setting stops and limits, etc. lens mapping is not currently offered but Chrosziel has been progressive about firmware updates and I am told they are working something up. I also remain in continuous contact with their team and if ever there is a question they promptly and personally respond, and Im in the USA, 6 hours ahead of them.
the only disclaimer to all of this is that i am a steadicam operator and not a focus puller. i have owned several follow focus and FIZ systems and so far this has been the most efficient. i have had a lot of ACs in New York, Los Angeles, and Atlanta really enjoy the experience of using it and I am happy to have it as a part of my kit.
please feel free to ask me any further questions. i am not on this forum much, but i am always on steadicamforum.com feel free to reach out to firstname.lastname@example.org